Extending Collinear Density Functionals to Noncollinear Cases under Periodic Boundary Condition

Abstract: Accurate modeling of spin-orbit coupling and noncollinear magnetism in materials requires noncollinear density functionals within the two-component generalized Kohn-Sham (GKS) framework, yet constructing and implementing noncollinear functionals remains challenging. Recently, a methodology was proposed to extend collinear functionals into noncollinear ones, successfully defining noncollinear functionals and their derivatives. However, the initial implementation involved a systematic approach to differentiate energy over density matrix elements rather than the derivatives of the energy functional with respect to density, presenting challenges for integration with periodic boundary condition-density functional theory (PBC-DFT) software. We have derived a novel set of working equations based on the original methodology, which provides noncollinear energy functionals and their derivatives. These working equations have been implemented in our noncollinear functional ensemble named NCXC, ensuring numerical stability and transferability without the need for incorporating derivatives of basis functions. This implementation is expected to facilitate compatibility with most DFT software packages. We demonstrate some preliminary applications in periodic systems, including noncollinear magnetism in spin spirals, band structures in topological insulators, and band gaps in semiconducting inorganic materials, using NCXC.